Mapping Mountains – Summit Relocations – 100m Twmpau

 

Graig y Barcut (SN 546 027) 

There has been a Summit Relocation to a hill that is listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Graig y Barcut (SN 546 027)

The criteria for the list that this summit relocation applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Graig y Barcut and this was derived from the Tithe map, and it is adjoined to the Mynydd Sylen group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with minor roads to its north, west and east, and the A4138 road to its south, and has the town of Pontarddulais towards the east.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list.

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 32m of drop, based on the 104m summit spot height positioned at SN 54707 02798 that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 72m bwlch height, based on interpolation of 5m contouring between 70m – 75m. 

Extract from the Ordnance Survey 1:25,000 Explorer map

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

LIDAR analysis gives the highest ground on this hill as 105.3m positioned at SN 54698 02765.  However, this is to the top of a raised field boundary and protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

LIDAR summit image of Graig y Barcut (SN 546 027)

The height produced by LIDAR analysis to the highest remaining natural ground of this hill is 104.0m and is positioned at SN 54698 02770, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is judged to be a relatively recent man-made construct, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the height produced by LIDAR analysis to the highest remaining natural ground of this hill is 104.0m and is positioned at SN 54698 02770, this is relatively close to where the spot height appears on the Ordnance Survey 1:25,000 Explorer map, and is approximately 5 metres northward from the high point of the raised field boundary. 

 

The full details for the hill are: 

Group:  Mynydd Sylen 

Name:  Graig y Barcut 

OS 1:50,000 map:  159

Summit Height:  104.0m (LIDAR) 

Summit Grid Reference (New Position):  SN 54698 02770 (LIDAR) 

Bwlch Height:  71.8m (LIDAR) 

Bwlch Grid Reference:  SN 54693 03117 (LIDAR) 

Drop:  32.2m (LIDAR) 

 

Myrddyn Phillips (November 2024)